LONDON (Reuters) - People who follow a Mediterranean-style diet rich in vegetables, fruits, nuts, whole grains and fish are less likely to become depressed, scientists said on Monday, but the reasons are unclear.

Spanish researchers studied 11,000 people and found that those who followed the Mediterranean diet most closely had a more than 30 percent reduction in the risk of depression than those whose diet had few of the crucial Mediterranean elements.

"The specific mechanisms by which a better adherence to the Mediterranean dietary pattern could help to prevent the occurrence of depression are not well known," said Almudena Sanchez-Villegas and colleagues at the University of Las Palmas de Gran Canaria and the University of Navarra, Spain.

But the researchers suggested that elements of the diet may improve blood vessel function, fight inflammation and repair oxygen-related cell damage -- all of which could reduce the chances of developing depression.

The study, published in the Archives of General Psychiatry journal, adds to an existing body of evidence showing the health benefits of a Mediterranean diet, including reduced risks of health disease, diabetes, asthma and cancer.

The study used data from Spanish people who reported their dietary intake on a food frequency questionnaire.

The researchers worked out how close their eating habits were to the Mediterranean diet based on nine components: A high ratio of monounsaturated fatty acids to saturated fatty acids; moderate intake of alcohol and dairy foods; low intake of meat; and high intake of legumes, fruit and nuts, cereals, vegetables and fish.

"Individuals who followed the Mediterranean diet most closely had a greater than 30 percent reduction in the risk of depression than whose who had the lowest Mediterranean diet scores," they wrote.